德尔福2010:令人困惑的泛型类型 TList 场景?按值传递还是按引用传递?
题
几天前,我在项目中使用 Generic TList 时遇到了一个问题。我在一个简单的测试项目中对其进行了测试,并遇到了同样的问题。这是确切的代码:
type
TMyPoint = record
x: Integer;
y: Integer;
end;
TShape = record
Corners: TList<TMyPoint>;
end;
TForm1 = class(TForm)
Button1: TButton;
Label1: TLabel;
procedure FormCreate(Sender: TObject);
procedure Button1Click(Sender: TObject);
private
Shape_List: TList<TShape>;
public
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
var
Shape: TShape;
FirstPoint: TMyPoint;
SecondPoint: TMyPoint;
Temp: TMyPoint;
begin
// Create the corners list
Shape.Corners := TList<TMyPoint>.Create;
// Add the first point to corners
FirstPoint.x := 10;
FirstPoint.y := 20;
Shape.Corners.Add(FirstPoint);
// Add the shape to the Shape_List
Shape_List.Add(Shape);
// Clear the shape corners
Shape.Corners.Clear;
// Add another point to corners
SecondPoint.x := 100;
SecondPoint.y := 200;
Shape.Corners.Add(SecondPoint);
// Show the x of the first point of the first shape
Label1.Caption := IntToStr(Shape_List[0].Corners[0].x);
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
Shape_List := TList<TShape>.Create;
end;
Label1.Caption 将是 100 不是 10 !为什么会这样呢?我想 TList.Add(const value)
是按值传递而不是按引用传递!
解决方案
在您的程序中添加了一些注释,以准确指出“错误”的位置。
procedure TForm1.Button1Click(Sender: TObject);
var
Shape: TShape;
FirstPoint: TMyPoint;
SecondPoint: TMyPoint;
Temp: TMyPoint;
begin
// Create the corners list
Shape.Corners := TList<TMyPoint>.Create; // We create a new TList<TMyPOint> OBJECT. Shape.Corners is a reference
// Add the first point to corners
FirstPoint.x := 10;
FirstPoint.y := 20;
Shape.Corners.Add(FirstPoint); // Add FirstPoint to the list we created at step 1.
// Add the shape to the Shape_List
Shape_List.Add(Shape); // Add a copy of the Shape record to the Shape_List
// Clear the shape corners
Shape.Corners.Clear; // Clear the Shape.Corners reference. This effectively clears the list of corners
// in the Shape you just added to Shape_List because it contains the same
// reference.
// Add another point to corners
SecondPoint.x := 100;
SecondPoint.y := 200;
Shape.Corners.Add(SecondPoint); // Add a new point to the Corners list. Remamber, Corners is actually
// a reference. The first Shape you added to Shape_List contains a copy
// of this exact same reference, so you're effectively adding a first point
// to both Shape.Corners and Shape_List[0].Corners.
// Show the x of the first point of the first shape
Label1.Caption := IntToStr(Shape_List[0].Corners[0].x); // Yup, you just added that point, so you get 100
end;
其他提示
添加后 FirstPoint
, , 你 Clear
这 Corners
(之后,列表为空)。然后你添加 SecondPoint
它成为第一个元素(索引 0)。
编辑:为了显示:
var
Shape1, Shape2: TShape;
begin
Shape1.Corners := TList<TMyPoint>.Create;
Shape2 := Shape1;
OutputDebugString(PChar(Format('Shape1.Corners: $%.8x', [Integer(Shape1.Corners)])));
OutputDebugString(PChar(Format('Shape2.Corners: $%.8x', [Integer(Shape2.Corners)])));
end;
Shape1.Corners
和 Shape2.Corners
指向同一个列表。
不,它是通过引用传递的,因为它是作为常量参数传递的(请参阅: http://docwiki.embarcadero.com/VCL/en/Generics.Collections.TList.Add).
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